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Michael J. Costello

Nodding needlegrass [Nassella cernua (Stebbins & R.M. Love) Barkworth], a California native perennial grass, was tested for its effects on grapevine and soil–water relations in a drip-irrigated vineyard in Parlier, CA. Vine water status and in-row and between-row soil moisture (at 0.3 m, 0.6 m, 0.9 m, 1.2 m, and 1.5 m) were monitored semiweekly from June to September. There was no overall significant difference in leaf water potential between treatments. In-row soil moisture was lowest at depths of 0.6 m to 0.9 m within the nodding needlegrass treatment but was lowest from 0.3 m to 0.9 m within the clean cultivation treatment. Compared with clean cultivation, nodding needlegrass in-row soil moisture was significantly higher at depths of 0.3 m and 0.6. m and did not differ at depths of 0.9 m and 1.2 m. In contrast, in-row soil moisture was significantly higher under clean cultivation compared with nodding needlegrass at 1.5 m. Between-row soil moisture was significantly higher under clean cultivation compared with nodding needlegrass at every depth. Combining in-row and between-row data, overall vineyard soil moisture was slightly lower, by 1.2% points, in the nodding needlegrass treatment compared with clean cultivation. There was no interaction between treatment and depth for between-row soil moisture, indicating that the vines used little water from the between-row area. The lack of difference between treatments in the rate of soil moisture depletion over the season indicates that nodding needlegrass used little water during the summer. Based on these results, nodding needlegrass appears to be suitable as a permanent cover crop in California drip-irrigated vineyards where competition for summer water is a concern.

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Michael J. Costello

Two California native perennial grasses, nodding needlegrass [Nassella cernua (Stebbins & R.M. Love) Barkworth] and California barley [Hordeum brachyantherum Nevski ssp. californicum (Covas & Stebbins) Bothmer, N. Jacobsen & Seberg], were compared with a conventional grass cover crop, ‘Blando’ brome (Bromus hordeaceus L.), as well as resident (weedy) vegetation and a clean cultivated control for effects on growth and yield of cultivated grape (Vitis vinifera L. cv. Barbera). Statistical analyses did not reveal yield differences between treatments with floor vegetation (the native grasses, ‘Blando’ brome, and resident vegetation) and clean cultivation, the cover crop treatments (the native grasses and ‘Blando brome’) and clean cultivation, nor the native grass treatments versus treatments with non-native floor vegetation (‘Blando’ brome and resident vegetation). However, there was a significant difference between the two native grasses with the average yield of nodding needlegrass 26.2% higher than that of California barley. Treatments did not differ in °Brix, berry weight, or pruning weight. At the end of the study, vine trunk diameter was 7.1% higher under the cover crop treatments than resident vegetation. Given these results, in vineyards where a neutral effect on growth or yield is desired, nodding needlegrass would be suitable as a permanent cover crop, whereas California barley would not.

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Michael J. Costello and W. Keith Patterson

Regulated deficit irrigation (RDI) is a management strategy that on grape can improve shoot/fruit ratio, water efficiency, and wine quality but has the potential to reduce yield. As part of a study on the influence of RDI on leafhopper density, we evaluated the effects on grape yield, berry size, berry soluble solids, and wine color. The studies were conducted at commercial vineyards in the San Joaquin Valley and in the Paso Robles region, CA, with Cabernet Sauvignon as the cultivar. Water deficits were imposed at either 50% (moderate deficit) or 25% (severe deficit) of standard irrigation (the control) for a period of 3 or 6 weeks and initiated at berry set, leafhopper egg hatch, or veraison. Deficit irrigation decreased berry weight by 16.1% at the San Joaquin Valley site (Aliso) and 11.7% at one of the Paso Robles sites (Frankel) but did not differ at the other site (Steinbeck). Yield was decreased by the deficits by 18.1% at Aliso, 26.7% at Frankel 2001 (but not 2002), and 24% at Steinbeck. Wine color density was increased by 21.8% at Aliso, 34.4% at Frankel 2001 (but not 2002), and did not differ at Steinbeck. Soluble solids did not differ among treatments at any site. There was no difference in berry weight, yield, or color between the moderate and severe deficits. It appears that in central California, RDI such as these are likely to reduce yield but are only one factor among many variables affecting quality such as wine color.